A stick-type ignition coil has a coil part provided in the middle part in the axial direction, a control part provided at its one end (upper end part) and a tower part at its other end (lower end part). In the coil part, a central core, a primary coil, a secondary coil and the like are accommodated within a cylindrical coil case. In the control part, an electronic control circuit (igniter) is accommodated within a box-shaped upper case. The coil case and the upper case are coupled to each other after being molded separately. The igniter causes the secondary coil to generate a high voltage by interrupting a current, which flows in the primary coil.
It is required to suppress an on-voltage to accurately control an ignition time point by the ignition coil. The on-voltage means a voltage, which arises in the secondary coil in the opposite direction to a predetermined voltage when the supply of current to the primary coil changes from the truned-off condition to the turned-off condition. This on-voltage arises at time points other than the ignition time points (primary current turn-off time points). Therefore, an engine will not operate normally when a spark plug generates sparks in response to this on-voltage.
It is possible to restrict the adverse effects due to the on-voltage by suppressing the voltage level of the on-voltage to be less than a voltage with which a spark plug generates sparks. In the conventional ignition coil (U.S. Pat. No. 6,095,125, U.S. Pat. No. 6,032,658 and JP10-176647A), for instance, as shown in FIG. 5, a battery voltage is applied to one end 81 of a primary coil 80 and a switching power transistor 82 is connected to its other end. A Zener diode 85 is connected between the primary coil 80 and a low voltage side of a secondary coil 84 connected to a spark plug 87 in such a manner that the on-voltage becomes a reverse voltage. The Zener diode 85 and a power transistor 82 are mounted on a circuit board of an igniter of a control part.
The diametrical dimension of the control part of this conventional ignition coil is large and hence the head part of this coil is expanded. Specifically, as the power transistor and the Zener diode are normally arranged on the circuit board of the igniter, the transverse area (planar area viewed as a plane) becomes large. Thus, the size of the igniter affects the size of the upper case, that is, control part.
The outer diameter of the coil part is required to be reduced to cope with the reduction of inner diameters of the plug holes in an engine head. It becomes difficult, however, to reduce the volume and diameter of the coil head part, because the size of the igniter affects the dimension of the coil head part. Thus, the reduction in volume and diameter of the coil part is limited in the conventional ignition coil, in which the on-voltage suppressing Zener diode is mounted on the circuit board of the igniter.